So, for backstory, I have a "Prehistoric Island" which has three zoos each dedicated to a sub-set of prehistoric life - a dinosaur zoo, a prehistoric "Sea World" aquarium and an Ice Age safari park. Gastornis is kept in the safari park.
A pair of Gastornis live in a large wooded enclosure that is largely modelled on the forest seen in the first episode of "Walking with Beasts", an episode Gastornis was also part of. As such, there are plenty of trees, plus some open areas for viewing purposes. A designated dirt patch is set aside for the birds to have a dust bath if they need one. An off-show breeding section, set-up in a similar way to the main exhibit, is also present. The Gastornis are given coconuts as regular enrichment. They were initially kept in the main safari reserve, but were moved to their own enclosure after they acted aggressively towards other animals, especially the park's free-roaming dodos.